1. Field of the Invention
This invention relates generally to a method and device for use in optical fiber signal transmission system. More particularly, this invention relates to a method and device for providing a variable optical attenuator.
2. Description of the Prior Art
In fiber optic communication systems, variable optical attenuators are broadly employed to regulate the optical power levels to prevent damages to the optical receivers caused by irregular optical power variations. As the optical power fluctuates, a variable optical attenuator is employed, in combination with an output power detector and a feedback control loop, to adjust the attenuation and to maintain the optical power inputted to a photo-receiver at a relatively constant level.
Variable optical attenuators can be generally classified into two types, namely, mechanical and non-mechanical types. In the mechanical type, the optical power levels are regulated by mechanically moving relative positions between some key optical components. The mechanical type variable optical attenuators have the disadvantages that such attenuators consume large amount of electrical power and have limited lifetime. In the non-mechanical type, regulation of optical power levels is achieved by changing the characteristics of optical components through electrical voltage or current adjustment Compared to the mechanical type, the non-mechanical type has longer lifetime and is more broadly employed in fiber optic communication systems.
FIG. 1 shows the structure of a type of non-mechanical variable optical attenuators. This type of variable optical attenuators include an input optical collimator 15, an input birefringent polarizer 25, a liquid-crystal polarization modulator 50 with a control voltage source 55, an output birefringent polarizer 60, and an output optical collimator 70. In this typical variable optical attenuator, an optical signal 20 coming from the input optical collimator 15 is spatially separated by the input birefringent polarizer 25 into two orthogonally-polarized beams 30 and 35 by employing the birefringent walk-off effect. The horizontally polarized beam 30 is indicated by its polarization direction 45 and the vertically polarized beam 35 is indicated by its polarization direction 40. Then the liquid-crystal polarization modulator 50 intercepts both the two orthogonally polarized beams 30 and 35 and rotates their polarization directions upon the voltage applied by the voltage source 55. At the exit plane of the liquid-crystal polarization modulator 50, the two beams 30 and 35 each carries a mixture of horizontal and vertical polarizations as indicated in FIG. 1. Note that the mixture ratio of the horizontal and vertical polarizations can be adjusted by changing the voltage applied on the liquid-crystal polarization modulator 50. Then both the beams 30 and 35 enter the output birefringent polarizer 60 having the same thickness as the input birefringent polarizer 25. The output-birefringent polarizer 60 spatially separates the two sets of the orthogonal polarizations carried by the two beams 30 and 35 into four beams. The mixture ratio of the four beams are determined by the mixture ration of the horizontal and vertical polarizations of the two original beams and can be adjusted by changing the voltage applied on the liquid-crystal polarization modulator 50. Finally, the vertically polarized component of the upper beam 30 is combined with the horizontal-polarized component of the lower beam 35 and then exits from the output birefringent polarizer 60 into the output optical collimator 70. The horizontal-polarized component of the upper beam 30 and the vertically polarized component of the lower beam 35 are guided to transmit away from the output optical collimator 70 and dissipated. As the optical power level at the output optical collimator 70 in this typical variable optical attenuator is determined by the mixture ratio of the four polarization beams, then the optical power level at the output optical collimator 70 can be electrically adjusted. Electrical adjustments can be achieved by adjusting the voltage applied on the liquid-crystal polarization modulator 50. The technical details of the typical attenuator that utilizes dual birefringent polarizers is more fully described in U.S. Pat. No. 5,963,291, entitled xe2x80x9cOptical Attenuation Using Polarization Modulation and a Feedback Controllerxe2x80x9d, by Wu et al. The disclosure made in U.S. Pat. No. 5,963,291 is hereby incorporated by reference in this Patent Application as part of the background of this invention.
While the typical variable optical attenuator as shown in FIG. 1 functions properly under most circumstances, it is limited by two major disadvantages. First, the typical attenuator employs two birefringent polarizers and thus has relatively high material cost. Second, since the typical attenuator employs two birefringent polarizers, polarization dependence loss often becomes a problem that adds to the uncertainties of the performance unless these two birefringent polarizers are perfectly aligned. In order to reduce polarization dependence loss, more stringent alignment requirements are implemented in the manufacture processes and thus cause the manufacture processes to be more time consuming and also more costly. Greater amount of time, efforts and costs are spent in manufacturing the typical attenuator due to the configuration that utilizes the dual birefringent polarizers. Due to a configuration implemented with birefringent polarizers, those of ordinary skill in the art for manufacturing the non-mechanical type of variable optical attenuators are still confronted with a difficulty that the material and assembly costs of the variable optical attenuator are quite high and cannot be easily reduced.
Thus, further development of new configurations in manufacturing the variable optical attenuators is required to provide attenuators that can be manufactured at lower material and assembly costs and with better polarization dependence losses. Particularly, a need still exists in the art of design of non-mechanical variable optical attenuators to provide new device structures without the use of birefringent polarizers is required to overcome the difficulties discussed above. By eliminating the use of birefringent polarizer, a non-mechanical variable optical attenuator with lower material and assembly costs and near-zero polarization dependent losses can be achieved.
It is therefore an object of the present invention to provide a new design for a non-mechanical variable optical attenuator that has near-zero polarization dependence loss and low material and assembly cost when compared with the conventional one as described above. By implementing the new and improved attenuator of this invention, the aforementioned difficulties in the prior art can be overcome.
Specifically, it is an object of the present invention to provide a non-mechanical variable optical attenuator arranged with a configuration to enable a self-interference between the segmented portions of a collimated beam. The self-interference is achieved by first generating a collimated beam, applying separate optical processes to the segmented portions of the collimated beam, and then inducing and controlling interference between the segmented portions. In this new non-mechanical variable optical attenuator, regulation of the optical power level is carried out by adjusting the degree of self-interference and that is in turn achieved by controlling the relative phase difference between the segmented portions of the collimated beam. The attenuation is generated without the use of birefringent polarizers. By eliminating the use of the birefringent polarizers, the variable optical attenuator of this invention can be produced at lower cost. The polarization dependence loss is maintained near zero. Therefore, the new and improved non-mechanical variable optical attenuator can be employed in fiber optic communications for broaden applications without being limited by the cost and polarization dependence loss problems as encountered in the prior art.
Another object of the present invention is to provide a simplified reflective type of a non-mechanical variable optical attenuator that can further reduce the material cost by employing a reflective mirror for reflecting an collimated composite beam with the phase difference back to the same lens. Cost savings are achieved by employing only one lens and one fiber pigtail.
Briefly, in a preferred embodiment, the present invention discloses a non-mechanical variable optical attenuator. The attenuator includes an input optical waveguide and a collimating means for collimating the optical beam from the input waveguide. The collimated beam is separable into an upper segmented-portion and a lower segmented-portion. The attenuator further includes a phase shifting means selectively generating a phase difference between the upper segmented-portion and the lower segmented-portion of the beam. The attenuator further includes a focusing means for focusing the collimated beam into an output optical waveguide. In a preferred embodiment, the phase shifting means further comprising a controller for controlling the phase shifting means for generating a variable phase difference. In a preferred embodiment, the phase shifting means comprising a polarization rotation means for rotating a polarization direction of the upper segmented-portion and an optical plate to pass the lower segmented-portion with no change of polarization direction to compensate the optical path-length difference between the upper segmented-portion and the lower segmented-portion. In a preferred embodiment, the collimating means comprising a GRIN lens for collimating the optical beam into a collimated beam. In a preferred embodiment, the collimating means further comprising a single optic fiber for receiving and projecting the optical beam to the GRIN lens. In a preferred embodiment, the focusing means further comprising an output single mode fiber and a second GRIN lens for focusing the expanded collimated beam into the output fiber.
Alternately, the invention also discloses a non-mechanical variable optical attenuator that includes a collimating means for collimating the optical beam into an collimated beam separable into an upper segmented-portion and a lower segmented-portion. The attenuator further includes a phase shifting means comprising an upper polarization means and a lower polarization means for receiving the collimated beam for generating a phase difference between the upper segmented-portion and the lower segmented-portion. The attenuator further includes a reflecting means for reflecting the collimated beam back to the collimating means for focusing the collimated beam into an output optical beam.